1. Field of the Invention
The invention concerns a method by which a polypropylene blend is produced by processing a plastic mixture that is composed of polypropylene and other high molecular weight polymers and which is contaminated with low molecular weight polymers and other impurities. Additionally, the invention concerns a polypropylene blend with specific characteristics produced in accordance with the method.
In this case, xe2x80x9chigh molecular weight polymersxe2x80x9d include such polymers having a molecular weight over approximately 10000, for example, polyvinyl chloride, polystyrene, polyethylene terephthalate, polypropylene and polyethylene such as, for example, LDPE (low density polyethylene), LLDPE (linear low density polyethylene), MDPE (medium density polyethylene), HDPE (high density polyethylene) UHDPE (ultra high density polyethylene) with corresponding degree of polymerization.
In contrast, xe2x80x9clow molecular weight polymersxe2x80x9d include such polymers having a molecular weight of up to approximately 5000, for example, waxes or other degraded high molecular weight polymers.
The plastic mixture may also contain polyester.
2. Background of the Invention
During the introduction of the Dual System in Germany, while implementing the packaging regulation, the proper recycling of the collected plastics posed a particular challenge. An entirely new fraction, namely the plastics composites, was introduced on the market. Special recycling methods must be found for the heterogeneity and varying composition of the composite plastics. Raw material recycling is principally appropriate for this purpose. Since the plastics, within the framework of the Dual System, are collected in conjunction with a range of interfering materials, methods have been developed that reduce the separation expense to a justifiable level, but that also ensure a specific degree of purity of the plastics material. The methods described in international patent application nos. WO 96/20819 and WO 99/36180 are examples of dry process methods for the generation of composite plastic agglomerate. The method in accordance with international patent application no. WO 98/18607 is also successful when polyolefin is extracted using, among other things, sink-float separation.
After all, the objective is to separate the composite plastics into specific types of polymers and to employ them in making new products. In this regard, a method is suggested in international patent application no. WO 00/77082 A1, which was published after the priority date of the present application, by which a polyolefin plastic fraction is brought into contact with a solvent and the temperature of the solvent and, as the case may be, also the ratio of solvent to the quantity of plastic is adjusted in such a manner that as many of the polymer types as possible are dissolved. In a subsequent solid-liquid separation, one polymer type is precipitated from the solution using shearing. PP (polypropylene), LDPE (low density polyethylene), HDPE (high density polyethylene) could, in particular, be separated in conjunction with a thermal separation method, for example, in accordance with German patent application no. DE 198 06 355 A1, in which two liquid phases are produced, one of which is solvent rich and a second is polymer rich.
The content of waxes, additives, decomposition products and other impurities interferes during further recycling, particularly when these polyolefins are intended for use as new products. German patent application no. DE 100 62 437.5 which shares the same priority date, suggests a method by which the plastic mixture is introduced into an organic solvent in which at least one of the high molecular weight polymers is soluble at a specific temperature. In this case, in the presence of the employed organic solvent, the specific temperature can be defined as the dissolving temperature at which at least one of the polymers contained in the plastic mixture will be present in dissolved form by more than 10-50% by weight. In the method, the solvent at normal pressure is maintained at an operating temperature at which none of the high molecular weight polymers is dissolved and whereby a specific weight ratio of plastic mixture to solvent is adjusted. During a certain residence period, the low molecular weight components are extracted and, if necessary, the plastic mixture could then be removed from the solvent.
Each of international patent application nos. WO 96/20819, WO 99/36180, WO 98/18607 and WO 00/77082 A1 and German patent application nos. DE 198 06 355 A1 and DE 100 62 437.5 is incorporated by reference as if fully set forth herein.
Surprisingly, it was determined that products with excellent material characteristics could be generated if, initially at temperatures lower than the dissolving temperature, the plastic mixtures are freed from soluble components using solid-liquid extraction (SLE) and only afterwards are dissolved as well as subjected to subsequent separation steps. This would be particularly successful if the plastic mixture is available in pellet form.
By xe2x80x9cpelletsxe2x80x9d is meant granulate or agglomerate which could be produced using the above mentioned dry and wet processing methods. Granulate and in particular agglomerate are characterized by high porosity or, as the case may be, by having surface fissures so that the extraction of low molecular weight polymers and other contaminants, surprisingly, is achieved with a satisfactory level of purity. Depending on the composition of the starting material, plastics blends are generated which are intended to be used together with new materials or as a replacement for new materials.
Of the plastics blends, the polypropylene blend is of particular interest.
Polypropylene is a part-crystalline material in which the crystalline share is between 50 to 70% depending on the production conditions. In this manner, isotactic, atactic and semi-tactic polypropylene is produced during propylene homopolymerization. The higher isotactic, or as the case may be, higher crystalline homopolymers can be subjected to heavy-duty mechanical pressure, are temperature stable and have low impact strength at low temperatures. The production processes may be differentiated based upon the different catalyst systems that are used for polymerization. Accordingly, a distinction is made between Ziegler-Natta and metallocene polypropylenes. Furthermore, different methods exist according to which block and random copolymers could be produced.
In Dr. Ing. Bodo Carlowitz""s manual xe2x80x9cKunststofftabellenxe2x80x9d [Plastics Tables], Hanser Publishing House, 4th edition, pp. 33 to 47, the common polypropylene characteristics are described. It reveals that the melt flow index MFR (measured at 230xc2x0 C. with a load of 2.16 kg in accordance with ISO 1133) of the polypropylene produced in accordance with the above method, is in the range of 0.2 to 50 g/10 min. With an increasing MFR value, the tensile strength of each polypropylene type decreases significantly so that the yield stress lies between 30 and 34 N/mm2 for a higher isotactical homopolypropylene at a density of 0.906 to 0.910 g/cm3 with a very high MFR value of 20 to 40 g/10 min.
The processing of polypropylene usually occurs using injection molding. To achieve faster cycle times but still maintain polypropylene injection molded parts with good constant mechanical characteristics, it is desirable to generate a polypropylene from plastic packaging materials that flows easily and which has mechanical characteristics which are as good as a polypropylene that flows less easily.
Therefore, it is the object of the invention to make available a method with which a polypropylene blend with a defined composition and a purity of more than 95% can be produced from mixed plastic packaging materials that as opposed to a new product has the advantage in the processing that it flows easily while retaining satisfactory mechanical characteristics.
These objectives are solved by the method in accordance with the invention and a polypropylene blend produced in accordance with the method of the invention.
The method in accordance with the invention is characterized by
the introduction of the plastic mixture into an organic solvent in which at least one high molecular weight polymer from the plastic mixture is soluble at a temperature specific for the polymer;
the maintaining of the solvent under normal pressure at an operating temperature at which no high molecular weight polymer is dissolved and adjusting the weight ratio of plastic waste mixture to solvent;
the extraction of low molecular weight components during a particular residence period of the plastic waste mixture in the solvent;
the dissolving of the plastic waste mixture already freed from low molecular weight components and the execution of a solid-liquid separation (SLS) for the removal of insoluble components such as paper, aluminum and other polymers depending on the type of solvent;
dissolving and phase separation, in other words, taking advantage of the presence of at least two liquid phases for the separation of different polymer compositions into fractions; and dissolving and selective precipitation of the polypropylene fraction using shearing or flow;
degassing of the polypropylene fraction; and
granulation of the polypropylene fraction for the polypropylene blend.
This sequence of steps ensures that the polypropylene is made available in the desired purity.